The present invention relates to mobile communication systems and, more particularly, to Code Division Multiple Access (CDMA) transceivers used for car telephones and portable telephone systems (cellular systems) using direct spread CDMA (DS-CDMA) systems.
As a cellular system using a CDMA system, what is described in North American Standards TIA/EIA IS-95 is well known in the art. In this IS-95, one frame (20 ms) divided into 16 slots having a time length of 1.25 ms, and the transmission power is controlled for each slot.
Specifically, the base station measures the reception quality for each slot, and compares the measured quality with a predetermined desired quality. When the measured quality is superior to the desired quality, the base station instructs the mobile station to reduce the transmission power. When the measured quality is inferior to the desired quality, on the other hand, the base station instructs the mobile station to increase the transmission power. As a transmission power level command to the mobile station, the base station transmits transmission power control bit data to the mobile station. The transmission power control bit data is multiplexed with communication channel data transmitted from the base station to the mobile station.
The mobile station separates the transmission power control bit data by de-multiplexing from the received multiplexed communication channel data. When the command from the base station is to increase the transmission power, the mobile station increases the transmission power of the next slot from the present (or prevailing) slot transmission power by 1 dB. When the command is to reduce the transmission power, on the other hand, the transmission power of the next slot is reduced from the prevailing slot transmission power by 1 dB.
In the cellular system using the CDMA system, unless data received from the individual mobile stations are of uniform quality, it becomes impossible for the base station to receive data of inferior quality, thus resulting in a system failure.
In the cellular system the mobile stations can be moved freely within the service area, and the quality of the received data is greatly varied in dependence on the transmission loss due to distance from the base station, shadowing due to interruption of the transmission line, and multi-path fading due to transmission of reflected wave s along a plurality of transmission lines. In order to cope with such data quality variations, the CDMA mobile station described in the IS-95 should accurately control the transmission power within in a range of 60 to 80 dB (of 1 dB or below, for instance).
In the reverse link (i.e., in increments transmission from the mobile station and received in the base station), one of four different bit rates, i.e., 9.6, 4.8, 2.4 and 1.2 kbps, can be selected for transmission and reception. Thus, variable rate transmission at 1/2, 1/4 and 1/8 of 9.6 kbps as basic bit rate, for instance, is realized by thinning down the transmission data to 1/2, 1/4 and 1/8, respectively. Specifically, one frame (20 ms) is divided into 16 slots having a time length of 1.25 ms, and the transmission is allowed in all the 16 slots when the basic rate of 9.6 kbps is selected for transmission, while it is allowed in 8, 4 and 2 slots per frame when the bit rates of 4.8, 2.4 and 1.2 kbps are selected, respectively, that is, no transmission is allowed in the other slots.
As shown above, in the transmission power control in CDMA, the reception quality per bit (i.e., Eb/Io: the ratio between reception power Eb and interference power Io per 1 Hz per bit) should be made constant.
In the reverse link described in the IS-95, the reception quality (Ed/Io) is controlled to be constant by varying the transmission slot number, i.e., the transmission on-off ratio, according to the transmission bit rate. However, the transmission on-off ratio is determined absolutely by the transmission bit rate, and not varied according to the transmission power.
A method of transmission power control in variable bit rate transmission is disclosed in a spread spectrum power transmission system shown in Japanese Laid-Open Patent Publication No. 5-102943. In this system, the spread rate is made variable according to the transmission bit rate, and the transmission power is controlled in reverse proportion to the spread rate. The transmission power per bit is thus made constant. However, the transmission is always made continuously irrespective of the transmission bit rate.
A transmission power control method is also disclosed in a spread spectrum communication system shown in Japanese Laid-Open Patent Publication No. 63-313932. In this system, the inter-transceiver distance is detected from the phase difference between spread code for transmission and that for reception. However, although the transmission power is controlled according to the detected distance, the transmission on-off ratio is not varied according to the transmission power.
In a system like the reverse link as disclosed in the IS-95, in which in a constant bit rate range the transmission power is increased or reduced in units of 1 dB without varying the transmission "on" time ratio or the spread rate, the transmission power of the mobile station should be controlled accurately in a control range of 60 dB (or 80 dB). Therefore, it is a problem that the design of the power amplifier (or transmitting amplifier) of the mobile station is complicated.
In addition, for linear operation of the transmitting amplifier with a broad dynamic range, it is necessary to operate the transistor of the transmitting amplifier with the same current as in the transmission with the maximum transmission power. Therefore, the power efficiency is inferior.